Method and apparatus for melting a metal

ABSTRACT

In a method of melting metal of a predetermined liquidus temperature, particularly a non-iron metal, such as magnesium, in a heated melting chamber into which solid metal is introduced and where a stream is generated, the parameters of flow are chosen such that the melting time is, in maximum, half the melting time without this stream under the condition that the temperature of the molten metal, when measured at at least one place in a distance of 5 mm in maximum from the solid metal, does not fall below liquidus temperature. To this end, an apparatus may be provided comprising at least one pump in a melting chamber having an associated heating device. This pump sucks the melt through at least one inlet opening and discharges the melt through at least one outlet opening. Both inlet opening and outlet opening are arranged within the melt bath of the melting chamber.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of melting metal,particularly a non-iron metal, such as magnesium of a predeterminedliquidus temperature in a heated melting chamber having a bottom,wherein solid metal is introduced, and wherein a stream is generatedwithin the melt. If, in the context of the present invention, it isspoken of a “metal”, this term should also concern alloys of any kind.

BACKGROUND OF THE INVENTION

[0002] Generating a stream of molten metal in a melting chamberaccording to the prior art is effected to various purposes. For example,WO 99/48637 provides an agitator for avoiding gravitational segregation.Although there was a “sluice” having a heating device for its own withinthe melting chamber and which was closed by a filter, but, of course,the stream had no influence to the interior of this closed recipient.

[0003] The construction according to DE-A-195 04 415 has also anagitator which, however, serves to inject an inert gas into the meltthrough its hollow shaft, to distribute this inert gas within the melt,thus driving oxides and other swimming matters to the surface level ofthe bath of molten metal.

[0004] In both cases, as above, the agitators are far enough from thesolid metal to be molten in order to allow effective agitating the meltof a small viscosity at all (as will be explained later on withreference to the recognition on which the invention is based) which isfar from the solid metal. It is clear that such agitators will also makethe temperature of the bath more uniform even with excessivetemperatures.

[0005] Pumps have also been incorporated into a melting chamber (e.g.U.S. Pat. No. 5,411,240), but they served to the purpose of conveyingthe melt from the melting chamber into a further chamber, i.e. they hadan inlet or sucking opening in the melting chamber, while the outletopening was directed towards this further chamber.

[0006] In order to be able to melt solid metal quickly in a meltingchamber without risking too high a heat loss therein, it is known topre-heat pigs which constitute the solid raw material. This, however,involves expenditure as to space, investment and energy. To achievequick melting, often excessive temperatures in the melting chamber areaccepted in practice. However, this results in a strong load of thelining of a furnace, of things built in and of the crucible itselfwhich, in turn, reduces the service life.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to achieve quick meltingwithout stressing the melting chamber and its parts excessively.

[0008] For solving this problem, the applicant has made intensiveinvestigations to better understand the mechanism of melting. From this,a recognitions resulted which led, in a first step, to the presentinvention. This recognition can be explained as follows.

[0009] In the art, as it was up to now, the melt, when introducing solidmetal, mostly in the form of pigs, has a reduced temperature in theimmediate surroundings of the solid metal still relative cold, partiallydown below the liquidus temperature, i.e. there will be an at leastpartial and, in any case, temporary solidification within this immediatesurrounding. According to the melting interval of the alloy, a more orless thick solidified layer will form itself around the pig. Shape andextent of this partially solidifies zone are dependent on the geometryof the metal to be molten and upon the original temperaturedistribution. Roughly spoken, partially solidified layers of differenttemperature will form around the solid metal body which, so to speak,wrap the solid body, thus preventing quick access of hot melt to theseinner layers.

[0010] On the other hand, there is still a further effect: The durationof the partially solidified state and the volume subjected to partialsolidification influence also the nature and amount of intermetalliccompounds forming within the melt. This means that both avoidingfreezing of the melt on the cold pig and quick melting thereof willessentially result to a large extent in avoiding formation of such, inprinciple undesirable, compounds and, therefore, to a better quality ofthe alloy. For the longer a certain volume is in a partially solidifiedstate, the greater is the amount of such compounds forming during thistime which contribute to an undesirable change of the alloy and to theformation of sump in the melting chamber and its crucible.

[0011] After these two recognitions had been gained, it was clear thatincreasing the melting rate was not only a question of productivity, butalso of the ultimate quality. Now, investigations were carried furtheron.

[0012] It has been found that virtually no convective heat transferoccurred within the partially solidified zone around the metal to bemolten. This is just what resulted in the phenomenon of a “wrap” ofpartially solidified metal around the solid metal. Within the partiallysolidified zone, convection exists only with a relative smalltemperature gradient, and the zone, as compared with the liquid melt,acts like a heat insulation. This explains also why the known agitators,used for other purposes anyway, could not change this condition, becausethey could only achieve a stream within a region of reduced viscosity,but not in the partially solidified zone of much higher viscosity.

[0013] This latter recognition of the influence of different viscositiesto the agitation effect should, actually, prevent those skilled in theart from generating a stream of molten metal within the melting chamberfor solving the problem. In so far, it is surprising when the inventorshad found that the above objects can be achieved if the parameters offlow are selected in such a way that the melting time is, in maximum,half the melting time without such stream, under the condition that thetemperature of said molten metal, when measured at at least one place ina distance of 5 mm in maximum from said solid metal introduced into saidmelting chamber, does not fall below said liquidus temperature.

[0014] This means that also in the case of the solution of the problemthere are two inventive steps: First, the stream has to have a certainflow energy or strength as well as a certain flow rate in the region ofthe solid metal. This, in turn, means that a certain directional effectshould be strived for, if one is not willing to generate an unusualstrong stream, as has never been strived for nor has been carried out upto now, in the whole bath of melt. This partial characteristic meansalso that it is advantageous to provide the stream generation device asclose as possible to the place where the solid metal is in the meltand/or that the flow energy and flow rate are selected in such a waythat it is sufficient even with a stream generation device at a certaindistance from the solid metal in order to achieve the criterion definedin the second partial characteristic, i.e. no reduction of temperatureof the liquid or molten metal below liquidus temperature.

[0015] Tests have shown that such temperature criteria can relativeeasily be measure by probes and that, as will be explained below, it iseven favorable within the scope of the present invention to provide atleast one temperature sensor in a distance from the heating device ofthe melting chamber (in order not to falsify its measuring value) withinthe direction of stream of the stream generation device, andparticularly of a pump used as such a device. Therefore, the term“pump”, as used in the context of the present specification should meanall devices that are able to generate a stream. The melting time canalso easily be determined by measuring once with generating such astream and once not.

[0016] As indicated above, it should be noted that the present inventionis not limited to a certain type of stream generation device, althoughat least one pump (in the narrow sense of this term, or more of them,are preferred. Alternatively, an agitator could be used which generatesan appropriately strong flow and, still better, an inductive agitatordevice, as is sometime used for continuous casting. With the latterdevice a stream directed towards the solid metal can easily begenerated. On the other hand, such inductive agitators are expensive andenergy consuming for which reason pumps (in the narrow sense of thisterm) are preferred.

[0017] In any case, comparison tests have shown that a one to ten timeshigher rate of melting the solid metal is possible using the methodaccording to the present invention. This is even the case, if oneconsiders the possibility of an entrainment of the solid metal in movingdevices provided to this end, as they are used in ultrasonic cleaning,particularly of optical glass bodies performing an up and down movement(or alternatively a circular carrousel-like movement). Tests have shownthat the formation of whirls generated necessarily is ratherdisadvantageous. A substantially laminar stream should be attempted,although will not be attained perfectly.

[0018] The measuring distance of 5 mm should, in fact, constitutes theadmissible maximum. Although it will be understood that a certainacceleration of melting would be achieved even if at least an extremethin layer is stripped from the “wrap” which surrounds the solid metal,but this would hardly be sufficient, due to the above-mentioned functionmechanism, to accelerate decisively the melting process. Preferably,however, the criterion in this connection, i.e. no reduction below theliquidus temperature, should be fulfilled already in a distance of 3 mmin maximum or even of about 1 mm.

[0019] A melting process in practice differs principally from the NewYears custom of lead casting. For it has to be considered that the rawmaterial supplied is subjected to various impurities which enter thebath during melting and should be separated there from pure moltenmetal. Part of these impurities will sink to the bottom due to theirspecific weight, another part, however, will form floating matter tofloat at the bath level surface. Therefore, it is preferred that thestream of molten metal has a main direction aiming away from the levelof the bath and/or (e.g. depending on the composition and kind ofimpurities) away from the bottom of the melting chamber. In this way,intermixing of impurities having already segregated to the bath level orto the bottom is prevented and, thus, any deterioration of the metal'squality. A main component of the stream which aims away (in the abovesense) will be best obtained if the stream of molten metal has asubstantially horizontal main direction.

[0020] The above explanation relating to the function of melting showsalso where, in some cases, a certain problem could reside when carryingout the method according to the present invention: a hot stream ofliquid metal is directed towards an object that, due to its partiallyfrozen state, has a significantly lower temperature. Of course, theeffect will be obtained that the stream of melt too will cool down.Particularly in a case where the flow of melt is circulated severaltimes, its melting effect can be clearly reduced. Therefore, it isadvantageous if the stream of molten metal, having passed the solidmetal, is directed against a heated surface of the melting chamber so asto be reheated again.

[0021] In order to be able to control or monitor the temperaturecriteria mentioned above, it is advantageous if the temperature in thestream of melt is measured, particularly downstream the solid metal.This measurement, however, can also be used to control heating of themelting chamber, although there are numerous variations possible: theoutput signal of a temperature sensor could also be used to control theenergy of the pump (flow rate and/or power) or a combination of suchcontrol possibilities would also be possible (e.g. first controlling oneof these parameters up to a certain limit and then the other parameter).It is preferred to control heating of the melting chamber in such amanner that the measured actual temperature is at least close to thenominal temperature, i.e. a non-liquidus temperature in a distance of 5mm in maximum, or (preferred) 3 mm in maximum, or (still more preferred)about 1 mm.

[0022] Bearing the above explanation of the term “stream generationdevice” in mind, it is preferred if an apparatus according to thepresent invention for melting metal of a predetermined liquidustemperature, comprises a melting chamber and an associated heatingdevice for heating the melt, and at least one pump having at least oneinlet opening and at least one outlet opening and a pumping elementbetween these openings for pumping said melt, wherein both the inletopening and the outlet opening are in the melting chamber and below thelevel of the melt therein. Thus the at least one pump sucks the melt outof the bath and returns it via the outlet opening. When speaking of an“inlet opening” and an “outlet opening”, it is to be understood that, ifdesired, a plurality of openings could be provided, e.g. several suctionnozzles and/or jet nozzles around the solid pig provoking a suctionstream and/or a jet stream. In this sense the term “opening” should beunderstood as meaning “at least one opening”.

[0023] To provide a stream of melt aimed directly towards the solidmetal, it is particularly advantageous if means are provided which forma defined deposit area for the solid metal. With such an arrangement, itis then preferred that at least one of the openings of the pump(s) isdirected towards this defined deposit area, particularly the outletopening. This deposit area may be defined by a holding device receivingthe solid metal and holding it in a distance from the bottom wall of themelting chamber. This has the advantage that the stream of melt canrinse around the solid metal from all sides, thus removing partiallysolidified or not completely molten layers of it into the bath of meltfor melting them completely.

[0024] One criterion according to the invention is that the stream ofmelt has such a flow energy and flow rate in the region of the solidmetal that a certain temperature condition id fulfilled. However, withdifferent metals and different melting temperatures, this may be verydifferent. Therefore, it is favorable if the pump is of the type havinga variable number of revolutions which may be varied by a manual orautomatic control unit.

[0025] In the preferred case of an automatic control unit, at least onesensor may be suitably provided for sensing at least one of theparameters of the temperature of said melt and the level of said melt.This sensor means will then provide a corresponding output signal. Forexample, the sensor may be a level sensor which is coupled to theautomatic control unit, while the output of this automatic control unitis coupled to the pump to control its number of revolutions. Othersensors used in the context of this invention may be a sensor for thesize of a pig, for the flow rate (which is quite different withdifferent viscosities), a “barrier” sensor which senses whether a pighas just been introduced into the melting chamber (then causing a highernumber of revolutions), optionally a timer (which reduces the numbers ofrevolution a time after the solid metal has been introduced and one cansuppose that it has been molten at least in part), or a sensor for thetemperature of a heated wall of the melting chamber (because the heatintroduced should reheat the stream having passed the pig, in apreferred embodiment). For example several ones of these parameterscould be input into a neuronal network (or a fuzzy control) weightingthem for providing a corresponding control signal, particularly forcontrolling the number of revolutions of the pump, but alternatively orin addition of the heating power and so on. From the above, it isapparent that it is advantageous if at least one of the sensors is atemperature sensor.

[0026] Above, an aimed stream towards the solid metal has been mentionedwhich can easily be achieved if a defined deposit area for the solidmetal is provided. However, regardless whether such a deposit area isprovided or not, an aimed stream may also be obtained by providing aguiding arrangement for guiding the flow of melt in a predetermineddesired direction. Such a guiding arrangement may then be used to directthe stream of melt in a direction as explained above, i.e. passing thesolid metal for removing partially solidified layers from its outersurface, on the one hand, but preferably away from the level of the bathand/or away from the bottom and/or against a heated wall to achievereheating of the melt after cooling by the solid and partiallysolidified metal.

[0027] If now a pump (in the narrow sense of this term) is used forgenerating the stream of melt, this pump may be of any kind. It can, forexample have its pumping element (rotor or plunger or injector) outsidethe melt so that the melt is supplied via a tube whose inlet opening isin the melt, while another tube or pipe returns then the melt into themelting chamber through the outlet opening. However, this would resultin undesirable cooling, for which reason it is preferred if not only theinlet opening and the outlet opening are in the melting chamber andbelow its melt level, but also the pumping element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Further details of the invention will become apparent from thefollowing description of embodiments schematically shown in thedrawings, in which

[0029]FIG. 1 is a cross-sectional view if a melting furnace according tothe present invention;

[0030]FIG. 2 is a cross-section along the line II-II of FIG. 1, butshowing a modified form of a holding stand for receiving a pig and ofits guiding surfaces; and

[0031]FIG. 3 a plan view, corresponding to arrow III of FIG. 1, of afurther modification.

DETAILED DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 shows a two-chamber-furnace 1 comprising a crucible 3inserted into a furnace space. The furnace 1 is subdivided along a planeP into a melting chamber 4 and an extraction chamber 5. A shieldingpartition or bulkhead 6 and a further transverse wall 7 below serve forthis subdivision. The transverse wall 7 has optionally a narrow opening8 for enabling a sump or bottom sludge to slide over the inclinedcrucible bottom 9 towards the melting chamber 4 so as to avoidcontamination of the melt in the extraction chamber 5. An opening 10serves the communication of the melt from one chamber to the other. Itwill be understood that this construction is only given by way ofexample and that the furnace may have any design desired.

[0033] Within the furnace space 2, a heating device 11 merelyschematically indicated is provided as is known per se. This heatingdevice 11 may be of any kind desired, for example it may be formed bythe pipes of a gas burner or by inductive heating coils. This heatingdevice 11, in the embodiment shown, does not only heat the bottom 9 ofthe crucible 3, but also at least one lateral wall 12. The upper side ofthe crucible is covered by a crucible cover plate 13 in order to avoidheat losses and/or the access of oxygen to the chambers 4 and 5. Thecrucible cover plate 13 may comprise connection pipes 14 of introducingan inert gas (such as argon) or a protective gas (e.g. nitrogen whichmay form magnesium nitride with magnesium) to cover the level surface ofthe melt with the inert or protective gas.

[0034] All these parts are of conventional nature and can appropriatelybe modified, if necessary. In the melting chamber 4 is a holding stand15 for depositing pigs 16, i.e. blocks of solid metal, which areintroduced into the melting chamber 4. By this holding stand 15, pigs 16are held in a distance from the bottom wall 9 so that the melt surroundsthe respective pig from all sides and transfers heat for melting it.

[0035] In accordance with the present invention, a flow or streamgenerating device, in the present embodiment a pump 18, is provided.This pump, in the embodiment shown, has a pump tube 19 in which a shaftextending along an axis A is supported which drives a pump rotor 20(merely shown in dotted lines) at its end. This pump rotor 20 sucks meltthrough suction or inlet openings 21 located within the melting chamberand below the ordinary level of the melt bath (which is mostly definedby the level at which the heating device 11 terminates). In this way,the pump rotor 21 generates a suction stream corresponding to arrows a1.The pump rotor 20 is driven by a motor 24 at the top of the pump 18which has, preferably, a variable number of revolutions.

[0036] The lower end of the pump tube 19 terminates in an outlet opening22 also being situated within the melting chamber 4 and, preferably,below the level of the melt bath. The outlet opening 22 is preferablydirected towards that area where the pigs 16 are accommodated at theholding stand 15 so that a direction of flow or stream corresponding toarrows a2 will form. Certainly, it would be conceivable to orient theoutlet opening 22 in a different direction so that the stream of meltemerging from it reaches the pigs 16, so to speak, according to the“billiard principle”, i.e. indirectly, but this will, in general, bemore energy consuming. Preferably, a guiding sheet 23 is provided abovethe suction or inlet openings 21 to avoid that suction has an effect tothe level surface of the bath. A similar guiding sheet could be providedbelow the suction openings 21 to avoid a “short circuit effect” directlyto the suction openings 21 of the melt stream exiting the outlet opening22.

[0037] Furthermore, it should be noted that any other stream generationdevice instead of an ordinary pump 18 could be used, such as amechanical agitator or an inductive agitator accommodated, for example,outside the crucible 3 or in its lining, but, in general, the efficiencywill be less. Likewise, the pumping element, in this embodiment the pumprotor 20, could be arranged outside the crucible 3, if only (and atleast) the inlet and outlet openings 21 and 22 are within the meltingchamber 4, but this would involve that the pumping element is at a lowertemperature level which is less desired.

[0038] The stream a2 is, as shown, directed towards that area where thepigs 16 are situated, i.e. slightly above the holding stand 15. It wouldbe conceivable to arrange it so that the suction stream a1 provided fora intensive rinsing of the pigs 16, but it is preferred, if this is doneby the pressure stream a2. The number of revolutions of the rotor 20,the proximity of the outlet opening 22 to the pigs 16 and the flowenergy provided by the motor 24 are selected in such a manner that themelting time is, in maximum, half the melting time without said stream,i.e. with the pump 18 switched off, under the condition that thetemperature of the molten metal, when measured at at least one place ina distance of 5 mm in maximum from said solid metal 16 introduced intosaid melting chamber, does not fall below said liquidus temperature. Thereason for this measure has already been discussed above.

[0039] Advantageously, at least one temperature sensor 25 will bemounted on the crucible cover plate 13 and will suitably be arranged ina distance D from the pigs 16, but also in such a distance from thelateral wall 12 that its heating device 11 substantially does not affectits measuring result. Distance D should conveniently be chosen 0.5 cm inmaximum, but preferably somewhat less, e.g. 0.3 cm in maximum. Since thestream a2 will be cooled when passing the cold pig 16, its temperaturewill be the lowest at the end of the pig 16, i.e. in the region of thelower tip of the temperature sensor 25. Therefore, if the temperaturemeasured at this point is above the liquidus temperature, one cansuppose that at least the greater part of the surface of the pig 16, ina distance of 0.5 cm in maximum, is above the liquidus temperature.

[0040] Therefore, it is advantageous to monitor this temperature by atleast one temperature sensor 25. If the temperature measured there istoo low, one could change the parameters of flow, i.e. for example, thepump 18 and its outlet opening 22 is approached to the pig 16. As analternative or in addition, the temperature of the heating device 11 maybe raised. The latter is particularly effective, because stream a2, asshown, is directed also towards the heated lateral wall 12 so that itflows, after cooling by the solid metal of the pig 16, along the lateralwall 12, thus gaining in temperature anew. However, the arrangement canbe different, because it would also be possible to direct the stream a2towards top of the pig 16, i.e. from above, so that the stream, havingpassed the solid metal 16, touches the heated bottom wall 9 to bereheated anew. Combinations of both stream directions would also bepossible.

[0041] However, it is preferred if, as mentioned above, the number ofrevolutions of the pump 18 (or the pump frequency of a plunger pump) canbe adjusted so that the flow rate of the stream can be controlled orinfluenced. This adjustment may, of course, be made manually, but it ismore advantageous if a control circuit, e.g. including a processor 26,such as a micro-processor, which is coupled with at least onetemperature sensor 25 to receive its output signal. If a plurality oftemperature sensors are provided, the processor 26 may weight the outputsignals thereof, the weight of the signals being, for example, in theorder of the stream direction of stream a2 so that the signal of thetemperature sensor 25 situated at the end has the highest weight,whereas the signals of sensors situated before have a lower weight.Weighting, as has been mentioned before, can be done by installing aneuronal or a fuzzy network in the processor 26.

[0042] In the present embodiment, the processor 26 has two outputs. Onthe one hand it is coupled to the motor 24 (or a final control stage notshown) for varying the number of revolutions of the pump 18. On theother hand, an output line is coupled to a final control stage 27 foradjusting the heating power of the heating device 11. For example,control in cascade would be conceivable where first the number ofrevolutions of the pump 18 up to a maximum (which may optionally be set)and, if the temperature according to the output signal of thetemperature sensor 25 is still too low, the heating power of the heatingdevice 11 is increased. Furthermore, a level sensor 32 (here symbolizedas an acoustic level sensor operating according to the echo principle,such as an ultrasound sensor, which, however, may be of any kind) whichcontrols the number of revolutions of the pump 18 through the processor26.

[0043] If the stream generated by the pump 18 is to circulate around thesolid metal 16, it is advantageous to provide guiding surfaces, besidesthe guiding surface 23 on the pump, also in the region of the pig 16 inorder to ensure that the (almost laminar, but whirls not being fullyexcluded) stream passes really along the surface of the pig 16 andagainst the heated wall 12. To this end, it is favorable if, forexample, the holding stand 15 is provided with appropriate guidingsurfaces 28, as illustrated in the embodiment of FIG. 2. It should benoted, however, that the present invention is not restricted to standssitting on the bottom 9, but also basket-like stands hanging from thecrucible cover plate 13 could be used.

[0044] In the embodiment of FIG. 2, the pig 16 is on vertical guidingsheets 28′ which form flow channels 29 between one another whereby thesolid metal of the pig 16 is “rinsed” or supplied with hot melt frombelow when the pump 18 is operating. The output opening 22 of the pump18, in this embodiment, is oval in cross-section so that the pig 16 ispassed by hot melt on its sides too. However, the output opening 22 isfar enough from the pig 16 that the exiting stream reaches also the flowchannels 29. It will be understood that such an arrangement having flowchannels that are open tworads the pig 16 could also be provided atother surfaces of the pig 16.

[0045]FIG. 3, in turn, shows an embodiment where the surfaces of theguiding sheets 28″ are parallel to the surfaces of the pig 16.Accordingly, the stream a2 enters flow channels 29 a, 29 b and 29 cformed between three sheet metal guidances 28″ parallel to each other.In the course of the path of flow, the stream of melt may cool down atthe solid metal 16. In order to guide new hot melt to the outer surfaceof the metal 16 to be molten, the flow channel 29 a is narrowed to asmall opening 31, and the middle flow channel 29 b carrying still hottermelt is directed to the pig 16. At the same time, the outermost channel29 c is deviated into a middle path where previously the flow channel 29b had been. In order to introduce simultaneously new hot melt a kind ofinjector opening 30 may be provided, leading into the flow channel 29 c.After a further third of the way, the outer flow channel 29 c isdirected towards the outer surface of the pig 16, an opening 31 beingprovided for allowing the melt conveyed through the channel 29 b to passthrough. These narrowings provoke also an increased flow rate whichcontributes to faster melting.

[0046] Although this arrangement has been shown in a plan view, it willbe understood that similar arrangements could either be provided for allouter surfaces of the pig 16 or only a part of them. It would also beconceivable to convey the stream of the respective channel passing alongthe outer surface of the pig 16, such as of channel 29 a, away in upwardor downward direction or laterally. In any case, it is essential thathot melt is directed to the outer surface of the pig 16 again and againover the length of the pig 16 either by utilizing an injector effect orin another way. For in each case a direction is associated to the streama2 by means of the guiding devices 28, 28′, 28″ which is directed awayfrom the level surface of the melt bath and/or from the bottom 9 of themelting chamber 4.

[0047] Numerous modifications are conceivable within the scope of thepresent invention; for example the holding stand 15 could be shiftabletowards the output opening 22 or away from it. Moreover, more than onepump could be used or, alternatively or in addition, a plurality ofoutlet openings 22 and/or inlet openings 21 around the pig 16.Furthermore, the holding stand 15 may be dimensioned so as to supportmore than two pigs 16. As has been mentioned above, the nature of thepump (or device having a pumping or entraining effect) is not decisive,for a plunger pump having a piston as a pumping element could be usedinstead of a pump, as illustrated, having a rotor as a pumping element(although the latter is preferred). Moreover, it is certainly favorableif the pig is lying in the melting chamber, but the principle of theinvention would, of course, also work providing vertically standing pigs(where the stream of melt is either vertical or horizontal or both).

What is claimed is:
 1. A method of melting metal of a predeterminedliquidus temperature in a heated melting chamber having a bottom, themethod comprising the steps of: providing a bath of said metal in moltenstate in said melting chamber, said bath having a certain level and atemperature above said liquidus temperature; introducing solid metalinto said bath so as to melt said solid metal during a melting time;generating a stream of molten metal in said melting chamber, said streamhaving certain parameters of flow including flow energy and flow rate;choosing said parameters of flow such that the melting time is, inmaximum, half the melting time without said stream under the conditionthat the temperature of said molten metal, when measured at at least oneplace in a distance of 5 mm in maximum from said solid metal introducedinto said melting chamber, does not fall below said liquidustemperature.
 2. Method as claimed in claim 1, wherein said metal is anon-iron metal.
 3. Method as claimed in claim 2, wherein said non-ironmetal is magnesium.
 4. Method as claimed in claim 1, wherein saiddistance amounts to 3 mm in maximum.
 5. Method as claimed in claim 4,wherein said distance amounts to about 1 mm.
 6. Method as claimed inclaim 1, wherein said condition is fulfilled at more than one place insaid distance.
 7. Method as claimed in claim 1, wherein said stream ofmolten metal has a main direction aiming away from said level of saidbath.
 8. Method as claimed in claim 1, wherein said stream of moltenmetal has a main direction aiming away from said bottom of said meltingchamber.
 9. Method as claimed in claim 1, wherein said stream of moltenmetal has a substantially horizontal main direction.
 10. Method asclaimed in claim 1, wherein said stream of molten metal is directedtowards said solid metal.
 11. Method as claimed in claim 10, whereinsaid stream of molten metal, having passed said solid metal, is directedagainst a heated surface of said melting chamber.
 12. Method as claimedin claim 1, wherein said at least one place is within said stream ofmolten metal.
 13. Method as claimed in claim 12, wherein said at leastone place is downstream said solid metal in said melting chamber. 14.Method as claimed in claim 1, wherein said temperature is measured tocontrol and maintain said condition.
 15. Method as claimed in claim 1,wherein said temperature is measured to control heating of said meltingchamber.
 16. An apparatus for melting metal of a predetermined liquidustemperature, comprising a melting chamber for receiving said metal in asolid state, said melting chamber having lateral walls and a bottomwall; heating means for heating at least one of said walls for providinga melt of said metal within said melting chamber up to a certain level;pump means having at least one inlet opening and at least one outletopening and a pumping element between said openings for pumping saidmelt, said inlet opening and said outlet opening being both in saidmelting chamber and below said level.
 17. Apparatus as claimed in claim16, wherein said pump means are of the type having a variable number ofrevolutions, said pump means further including control means for varyingsaid number of revolutions.
 18. Apparatus as claimed in claim 16,further comprising wall means forming an extraction chamber, andtransfer means for transferring melt from said melting chamber into saidextraction chamber.
 19. Apparatus as claimed in claim 16, wherein saidtransfer means comprise communication means between said melting chamberand said extraction chamber.
 20. Apparatus as claimed in claim 16,wherein not only said inlet opening and said outlet opening are in saidmelting chamber and below said level, but also said pumping element. 21.An apparatus for melting metal of a predetermined liquidus temperature,comprising a melting chamber for receiving said metal in a solid state,said melting chamber having lateral walls and a bottom wall; heatingmeans for heating at least one of said walls for providing a melt ofsaid metal within said melting chamber up to a certain level; pump meanshaving at least one inlet opening and at least one outlet opening forpumping said melt, said inlet opening and said outlet opening being bothin said melting chamber and below said level; and means forming adefined deposit area for said solid metal.
 22. Apparatus as claimed inclaim 21, wherein at least one of said openings of said pump means isdirected towards said means forming a defined deposit area. 23.Apparatus as claimed in claim 22, wherein it at least said outletopening that is directed towards said means forming a defined depositarea.
 24. Apparatus as claimed in claim 21, wherein said means forming adefined deposit area comprise holding means for receiving said solidmetal and holding it in a distance from said bottom wall.
 25. Apparatusas claimed in claim 21, further comprising at least one temperaturesensor arranged below said level and in a distance from both saidheating means and said means forming a defined deposit area. 26.Apparatus as claimed in claim 25, wherein said distance amounts to 5 mmin maximum.
 27. Apparatus as claimed in claim 25, wherein said distanceamounts to 3 mm in maximum.
 28. Apparatus as claimed in claim 25,wherein said distance amounts to about 1 mm.
 29. An apparatus formelting metal of a predetermined liquidus temperature, comprising amelting chamber for receiving said metal in a solid state, said meltingchamber having lateral walls and a bottom wall; heating means forheating at least one of said walls for providing a melt of said metalwithin said melting chamber up to a certain level; pump means having atleast one inlet opening and at least one outlet opening for pumping saidmelt, said inlet opening and said outlet opening being both in saidmelting chamber and below said level; and sensor means for sensing atleast one of the parameters of the temperature of said melt and thelevel of said melt, said sensor means providing an output signal. 30.Apparatus as claimed in claim 29, wherein said sensor means comprise atleast one temperature sensor arranged in a distance from said heatingmeans.
 31. An apparatus for melting metal of a predetermined liquidustemperature, comprising a melting chamber for receiving said metal in asolid state, said melting chamber having lateral walls and a bottomwall; heating means for heating at least one of said walls for providinga melt of said metal within said melting chamber up to a certain level;pump means having at least one inlet opening and at least one outletopening for pumping said melt, said inlet opening and said outletopening being both in said melting chamber and below said level; andsensor means for sensing at least one of the parameters of thetemperature of said melt and the level of said melt, said sensor meansproviding an output signal; and automatic control means receiving saidoutput signal and providing at least one control signal at their outputfor controlling at least one of the parameters including flow energy,flow rate, flow temperature and said level.
 32. Apparatus as claimed inclaim 31, wherein said sensor means comprise at least one temperaturesensor, said output of said automatic control means being coupled tosaid heating means for controlling their power.
 33. Apparatus as claimedin claim 31, wherein said sensor means comprise a level sensor, theoutput of said automatic control means being coupled to said pump meansto control them.
 34. An apparatus for melting metal of a predeterminedliquidus temperature, comprising a melting chamber for receiving saidmetal in a solid state, said melting chamber having lateral walls and abottom wall; heating means for heating at least one of said walls forproviding a melt of said metal within said melting chamber up to acertain level; pump means having at least one inlet opening and at leastone outlet opening for pumping said melt, said inlet opening and saidoutlet opening being both in said melting chamber and below said levelto generate a flow of melt; and guiding means for guiding said flow ofmelt in a predetermined direction.
 35. Apparatus as claimed in claim 34,wherein said guiding means are formed and arranged to direct said flowof melt away from said level.
 36. Apparatus as claimed in claim 34,wherein said guiding means are formed and arranged to direct said flowof melt away from said bottom wall.
 37. Apparatus as claimed in claim34, wherein said guiding means are formed and arranged to direct saidflow of melt in a substantially horizontal main direction.
 38. Apparatusas claimed in claim 34, wherein said guiding means are formed andarranged to direct said flow of melt first to said solid metal and thenagainst a heated wall of said melting chamber.
 39. Apparatus as claimedin claim 34, wherein said guiding means are formed at least in part onsaid pump means.